The present invention relates to a control system for a motor vehicle drive train including a continuously variable transmission.
Ratio control systems for continuously variable transmissions are well known in the art. Such a ratio control system is disclosed in co-pending U.S. patent application Ser. No. 411,987, filed Aug. 26, 1982 (now U.S. Pat. No. 4,515,040) which corresponds to European Patent Application No. 82 107 823.5. In this ratio control system, a target or desired reduction ratio is determined based on various input signals representative of varying vehicle operating conditions and a pulley unit, including a drive pulley, a driven pulley and a V-belt, is hydraulically controlled such that an actual reduction ratio is varied such that the deviation from the target reduction ratio is decreased.
Generally, continuously variable transmissions include a drive pulley, a driven pulley, a V-belt drivingly interconnecting such pulleys, a source of hydraulic fluid pressure, a pressure regulator valve for regulating the hydraulic fluid pressure to develop a control pressure variable with actual reduction ratio and engine's load (induction manifold), a hydraulic fluid network including a shift control valve establishing fluid interconnection among the regulator valve, drive pulley, and driven pulley for controlling fluid pressure supplied to the drive pulley relative to that supplied to the driven pulley so as to vary the reduction ratio in response to the position of an adjustable control member. The position of the adjustable control member is controlled by a microcomputer based control unit wherein a reduction ratio is varied such that the deviation of an actual reduction ratio from a target reduction ratio is decreased. The ratio control system of this kind is disclosed in co-pending U.S. patent application Ser. No. 543,838, filed Oct. 20, 1983 (corresponding to EP No. 83 110 546.5) now U.S. Pat. No. 4,576,265, issued Mar. 18, 1986.
In a ratio control system of the above kind, the deviation from the target reduction ratio is integrated and then multiplied with an integral gain to provide an integral control factor, and the deviation is multiplied with a proportional gain to provide a proportional control factor. The integral and proportional control factors are added to each other and, based on this result, a control unit generates a shift command signal to a shift actuator, such as a stepper motor, for adjusting the position of an adjustable control member.
Since the shift command signal is determined on the sum of integral control factor and proportional control factor, overshoot and hunting are caused when the deviation is large if the integral gain is set large enough to provide a fast response. On the contrary, if the integral gain is set small, there is provided a slow response. Therefore, the setting of the integral control gain is a compromise between drivability which requires fast response and stability which requires slow response.